It is well known that radio waves, propagating from a transmitter to a receiver, can follow a plurality of different paths and that the relative phases of the waves arriving at the receiving antenna can be such as to destructively interfere, causing what is commonly referred to as a fade. In order to reduce the opportunity for this to occur, the so-called "space diversity" system has been developed using two, spaced antennas to feed a common receiver. The theory underlying the use of two spaced-apart antennas is that there is less likelihood that a fade will occur at both antennas at the same time. In the simplest system, means are provided for disconnecting the receiver from the first antenna as soon as the received signal falls below a predetermined threshold level, and for connecting the receiver to the second antenna. In this so-called "blind switch" it is assumed that the signal received by the second antenna will be stronger than that received by the first antenna. In a more sophisticated system, the signals from the two antennas are combined at radio frequency instead of merely selecting the larger of the two. This eliminates amplitude and phase jumps associated with the switching operation, and has the added advantage of delivering a larger amplitude signal to the receiver. However, such a system requires the use of dynamic phase correction to compensate for variations in the relative phase of the two signals caused by changes in their path lengths. In one such system, described in U.S. Pat. No. 2,786,133, a single, continuously adjustable phase shifter is included in one of the antenna wavepaths and is automatically adjusted so that the wave from the one antenna has the proper phase to combine with the wave from the other antenna. U.S. Pat. No. 3,582,790 shows, in greater detail, a means for combining the two received signals and for isolating the two antennas from each other. The circuit includes a first phase shifter which shifts the phase of one of the input signals to bring it into quadrature phase relationship with the other. The quadrature related signals are combined in a first hybrid coupler to produce a pair of equal amplitude signals. The phase of one of the two signals is then shifted 90 degrees by a second phase shifter so as to bring the two signals in phase. The two equal, in-phase signals are then combined in a second hybrid coupler to produce a single output signal whose total power is equal to the sum of the powers of the two received signals.
Both of these systems seek to track the two signals continuously and do so by means of continuously variable phase shifters. The problem with such prior art phase shifters is that in order to go from maximum phase shift back to zero, it is necessary to go through all values therebetween. To illustrate the problem, consider two waves whose relative phase difference is slowly increasing. As the phase difference increases, it will eventually reach 360 degrees at which point the two signals are again in phase. However, a phase shifter such as the type illustrated in U.S. Pat. No. 2,786,133 does not ease past its maximum phase shift to zero phase shift but, instead, must be reset by going completely through its entire range of phase shifts from its maximum setting to its minimum setting, causing a sudden fluctuation in the amplitude of the output signal, including the possibility of signal cancellation. What is desired is a phase shifter which is capable of providing increasing or decreasing phase shifts without a return-toward-zero requirement.
The return-toward-zero problem is also present in other types of continuously variable phase shifters. For example, U.S. Pat. No. 3,419,823 shows a phase shifter comprising a tandem array of a 90 degree hybrid coupler and a 3 dB, 180 degree hybrid coupler. In this embodiment, the phase of the output signal is controlled by either changing the power division ratio of the 90 degree coupler, or by changing the attenuation in one of the two wavepaths connecting the two couplers. In either case, the return-toward-zero problem is not resolved by the phase shifter described in this patent.
The return-toward-zero problem can be avoided by using stepping phase shifters of the types disclosed in copending applications Ser. Nos. 578,528 now U.S. Pat. No. 3,993,050 and 878,561, now U.S. Pat. No. 4,153,994, filed concurrently on Feb. 17, 1978, wherein the signal phase can be advanced or retarded continuously in 90 degree steps. However, it is a limitation of this approach that phase correction is made in discrete increments and, hence, is only approximate. For example, the two signals can be as much as 45 degrees out of phase, resulting in some signal loss due to phase error. This can be avoided by using both a phase stepper and a continuously variable phase shifter of limited range in the manner described by K. L. Seastrand, Jr. in his copending application Ser. No. 905,156, now U.S. Pat. No. 4,160,952 filed May 12, 1978, and assigned to applicant's assignee.
One difficulty with all of the above-described phase shifters employing 90 degree phase steppers is that switching transients occur every 90 degrees. Advantageously, a phase shifter for use in a space diversity receiver would be capable of continuously variable phase shift; have no return-toward-zero problem; have fewer switching transients, less loss and increased bandwidth.